The field of this invention is centrifugal compressors featuring an integrally cast volute and more particularly to assemblies of compressors for multistage compression wherein the integral casting further includes a gearbox and intercooler housings.
Centrifugal compressors supply oil free compressed gas in a variety of industrial applications. A common application is for plant air systems to supply a motive force for valve actuators and pneumatic cylinders for use in robotic applications, as one example. Centrifugal compressors feature an impeller mounted in a closely conforming impeller chamber. The chamber features an axial inlet port to allow fluid entry toward the center of the impeller. Fluid is drawn into the impeller due to its rotation at speeds which can exceed 75,000 revolutions per minute (RPM). The rotation of the impeller propels the fluid through an annular diffuser passageway and into a surrounding volute. The energy imparted into the fluid by the impeller rotation increases the fluid velocity and consequently it pressure as the fluid passes the diffuser passageway and into the scroll or volute. The diffuser pas sage way has inside and outside radial dimensions for each circumferential station of the impeller chamber and scroll. By definition, the inside radius of the diffuser section corresponds to the distance to the diffuser throat or the location at which the annular port or passageway has the smallest axial width for the given station, the diffuser section extending outwardly for the remainder of the annular passageway.
In the past, centrifugal compressors have featured a bolt on scroll/volute cover, which encompassed portions of the impeller chamber, and the diffuser passageway and the volute outlet passageway. U.S. Pat. No. 4,181,466 is illustrative of a bolt on component featuring the fluid entry 51 and the volute 50, which is also secured to the bearing housing 15 by a V-clamp 49. One main problem with the bolt on scroll/volute cover incorporating the volute was the effective control of tip clearance between the impeller and the inlet passageway and the clearance between the impeller and the volute outlet. Due to the bolt-on construction previously employed, machining costs and assembly costs affected the finished cost of the product. The assembly of a plurality of components required the use of greater clearances around the impeller, which sacrificed compressor efficiency. This, in turn, required larger drivers and higher operating costs for electric power. Since each assembled component had a manufacturing tolerance, the final clearance near the impeller had to be sufficiently large to accommodate a situation where all the tolerances in the individual components of the assembly turned out within specification but all dimensions on the individual components were off from the ideal dimension and on the same side of the tolerance allowed.
Another problem with bolt on volutes, ie. 24 and 26, is the extra space and mass taken up by that type of assembly which could become important in situations where ease of installation and maintenance is important to serviceability. For example, as will be explained below when the preferred embodiment is described, use of bolt-on volutes (such as 24 and 26) precludes access to the driver shaft for an oil pump to be directly driven. The extra housing thickness for each stage in a multi stage skid could preclude a direct drive on the oil pump and may necessitate a separate electrical drive for the oil pump. This would be undesirable in the event of an electrical failure. In an electrical failure, the impeller bearings need lubrication as the impeller slows from its operating speed of 75,000 RPM or more. Bearing failure could result with an electrically driven oil pump because it would stop delivering oil too abruptly on power failure. A power takeoff from the main drive shaft, which could involve gears or belts adds to the complication of packaged systems and tends to complicate access when maintenance is required.
What is needed is a technique to obtain better efficiency from a centrifugal compressor, whether running alone or connected to others in a multi-stage compression application. One of the objects of the present invention is to realize efficiency and operating cost improvements by integrally casting the volute as a part of the gearbox. Another objective is the reduction of radial clearance on the impeller, which results in an improvement of its efficiency. Another objective is to reduce production and assembly costs. These and other advantages of the present invention will become more apparent to a person of skill in the art from a review of the description of the preferred embodiment described below.
A casting technique is disclosed which improves efficiency and reduces fabrication and assembly cost for a centrifugal compressor. The volute is cast integrally with the gearbox base to allow closer radial tolerances to be used to improve efficiency. In the preferred embodiment, compressors for multi-stage compression are assembled with intercoolers and the integral volute is cast together with the impeller housing and the lower gearbox housing and the associated intercooler. Efficiency increases of 2% or more are achievable. In multistage applications, efficiency gains in the early stages are compounded in each subsequent stage.